The research described here concerns the interactions between biological macromolecules in solution and in highly ordered structures. Two types of systems are of interest: a) those in which the interactions are limited to protein-protein interactions, and which give rise to small molecules of well- defined size; and b) those in which there are interactions between proteins and other substances, and which give rise to aggregates composed of large numbers of polypeptide chains. With regard to the first type, the specific area of interest is the correlation of the structure and function of protein molecules which are composed of more than one polypeptide chain and which exhibit cooperative interactions (or any type of linkage) between the polypeptide chains in the whole molecule. The second aspect concerns the way in which protein molecules are involved in the structure and function of cell membranes and of microtubules. This work should provide the basis for understanding several biological phenomena: the regulation of enzyme activity, the function of cell membranes (especially with regard to their permeability properties, and to their interactions with hormones), and the mechanism of assembly of intracellular microtubules.